The curing of coating systems which carry activated double bonds by actinic radiation, such as UV light, IR radiation or else electron beams, is known and is established in industry. It is one of the most rapid curing methods in coating technology. Coating compositions based on this principle are referred to as radiation- or actinic-curing or -curable systems.
Very simple, flexible urethane acrylate prepolymers can be prepared by reacting NCO prepolymers based on a polyester, polyether or polycarbonate polyol with an OH-functional compounds containing activated double bonds, such as hydroxyalkyl acrylates, for example (P. K. T. Oldring (Ed.): Vol. 2 “Prepolymers and Reactive Diluents for UV and EB Curable Formulations”, 1991, p105ff). Depending on the nature and the molecular weight of the polyol used it is entirely possible in this case for very flexible and elastic binders to form that are curable by actinic radiation. A disadvantage here, however, is the typically high viscosity of such systems, which frequently necessitates the use of reactive diluents. Moreover, with a very high molecular weight, such systems are not of very high functionality, which impacts adversely on the reactivity and the resistance towards solvents.
A significant advantage in terms of viscosity is offered by systems of the kind described in, for example, the European application with the application number 06004171.2. There the urethane groups present in the prepolymer are reacted with monoisocyanates to form allophanates, which significantly lowers the viscosity of the binders. The problem of the low functionality and hence low resistance, however, is not addressed.
On the other hand, however, the method of allophanatization as described in GB 994,890 or EP 682 012, for example, is entirely suitable for raising the functionality of a binder. Applications WO 2005/097737 or WO 2005/097865, for example, describe prepolymers based on difunctional polyethers which are reacted by urethanization and subsequent allophanatization to give tetra-functional NCO prepolymers. A further reaction with hydroxyalkylacrylates would lead, however, to a very awkward operation with little advantage, which as well as two separate reaction procedures would also involve distillation of the excess diisocyanate.
It is likewise possible to react an acrylate-containing and NCO-containing allophanate, as described in EP 1 144 476 B1, in an additional step with further hydroxyalkyl (meth)acrylate and a polyester, polyether or polycarbonate polyol.
Although this procedure again gives the polyfunctional prepolymers described above, it produces a saving in terms neither of the distillation of the products nor of one of the two reaction procedures. In addition, this operation harbours the difficulty that the distillation step must take place at temperatures up to 135° C. in order to allow the residual isocyanate content to be lowered sufficiently (<0.5% by weight residual monomer). There is a risk that, even during the purifying operation, double bonds will react under thermal initiation in polymerization, with the consequence that ideal products are no longer obtained.
It is also possible to prepare allophanates indirectly, from isocyanate derivatives other than urethanes and isocyanates. For instance EP-A 0 825 211 describes a process for building up allophanate structures from oxadiazinetriones; a further route is the opening of uretdiones (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419 and also US-A 2003 0153713) to give allophanate structures. In both cases the use of polyester, polyether or polycarbonate polyols leads either to the ring opening or to the target products even at the urethanization stage. Both routes, however, require high-grade raw materials as their starting material and lead only to an allophanate product which is rich in by-products.
EP-B 694 531 describes a multi-stage process for preparing hydrophilicized allophanates containing radiation-curing groups. In that case, however, first an NCO- and acrylate-functional urethane is prepared, which is hydrophilicized and subsequently allophanatized following addition of a further NCO- and acrylate-functional urethane. As the process temperature for the allophanatization, very high temperatures of 100 to 110° C. are specified, which may lead to an unwanted free-radical polymerization.
In the European application with the application number 05020859.4 a process has now been described that allows the preparation of radiation-curing, allophanate-containing binders based on easily available raw materials, such as polyisocyanates and polyols, in an operation already at a moderate temperature of below 100° C. without a distillation step. The coatings produced from these coating materials are very hard, but their elasticity and solvent resistance are each in need of improvement.